In a peer-to-peer (P2P) file-sharing system, the faithful delivery of an authentic file depends on the authenticity of the file advertisement as well as the authenticity of the shared file advertised. We present the index authenticity problem in a distributed P2P indexing scheme, and propose to employ a secure index verification scheme, which allows a querying requestor to securely verify the authenticity of file advertisements in the query response from a distrusted index peer with unforgeable proofs of indexing correctness, in order to combat index poisoning attacks targeting index authenticity. A solution based on signature and bloom filter (BF) and its cost-efficiency analysis are given.